The present invention relates to protection devices for erasable and reprogrammable read only memories for integrated circuits.
The first type of erasable and reprogrammbale ROM is known as EPROM memory. It is a non volatile memory whose contents may be erased by exposure to ultraviolet radiation. Without voluntary erasure of the contents of such a memory, it is always possible to write on top of a first program, but that generally results in superimposing two programs, that is to say in short in losing the information.
The second type of erasable and reprogrammable ROM is known as EEPROM memory. In this case they are non volatile memories whose contents may be erased electrically, that is to say by applying predetermined voltage levels. In this case as well it is always possible to reprogram on top of the first program, but that also results in losing the information.
When such memories have been programmed, it is not possible to know if they are effectively programmed without reading their contents.
Now, for reading the contents of a memory, it must be connected to a reading circuit. This operation is tedious but necessary if, when programming a memory, it is not desired to superimpose two programs which would consequently lead to information no longer having any meaning or to a loss of the first information stored.
It is known to protect EPROM and EEPROM memories against any falsification of their contents by making them unusable. For that, a tell tale bit is programmed into the memory, at the end of programming thereof. Thus, when the memory is deprogrammed, voluntarily, or involuntarily, the tell tale bit is also deprogrammed. Since this tell tale bit is used for addressing during reading, reading of the memory is blocked. The presence of the tell tale bit therefore prevents the use of the memory only when it has been partially erased or reprogrammed. Such a protection is not suitable for solving the problem which is raised, since it is a question of preventing undesired reprogramming.